Oscillator control system



Feb. 24, 1959 Filed April 27, 1956 H. A, ROBINSON oscILLAToR coNTRoLSYSTEM 2 Sheets-Sheet l F6524, 1959 H. A. ROBINSON l o'sCILLAToR CONTROLSYSTEM 2 Sheets-Sheet 2 Filed April 27, 1956 INVENTOR. HAM/s A. Roem/50Nf77-Tamm `other oscillator at a` harmonic of the `series of frequencydividers. 4 uthe last, in addition to the wave fed theretofrom theUnited OSCILLATR `CONTRL SYSTEM Harris A. Robinson, Palmyra, N.

J., assignor to Radio "Corporation of America,

a corporation of Delaware Application April 27, 1956, Serial No. 581,1558 Claims. (Cl. Z50- 36) This invention relates to an oscillator controlsystem, and more particularly to `a frequency control system forstabilizing and controlling` the frequency of a master oscillator of thecaptive type.

This invention constitutes an improvement over my frequency `and theoutput of which in turn is used for A lockingin the master (captive)oscillator. Each save the last of the aforementioned mixers is also fedwith a` respective stable reference frequency wave derived by frequencydivision froma referenceV crystal. However, in the first of theaforementioned applications the reference frequency wave fed to the lastmixer is derived from a selected `pair of crystals which are separatefrom the reference crystal.

The second `of the aforementioned applications simplies the arrangementof the r'st application in that the extra crystals which provideheterodyning input tothe last mixer are eliminated, all of the referencefrequency Waves fed to the respective mixers then being derivedfrom asingle reference crystal. This simplification is effected arrangement, astabilized oscillator controlled by pulses derived from the single.reference crystal. In said second application, these controlling pulsesare generated by a pulse generator `excited by a stable frequency waveobtained by frequency division from the single reference crystal. z i

An object of the present invention is to devise an im* proved frequencycontrol system of enhanced `stability for a multichannel oscillator ofthe captive type.

'Another object is `to devise a frequency control system` employing apulse generator, in` which such pulse gen-` erator is used both to lockin an` oscillator at a subharmonic of a reference frequency (thus` ineffect dividing downthis reference frequency), and also `to lock inanfrequency of the first oscillator. v

The objects of this invention are accomplished, briey, in thevfollowingmanner: for the frequency control of `a multichannel master (captive)oscillator, a sample of the output of this oscillator is fed into theiirst'of a plurality of mixers arranged in cascade. In order to providea plurality of stable reference frequency waves, the output of a singlecrystal oscillator is fed through a Each of the mixers save precedingmixer or from the multichannel master oscilies Paten 'icc lator, issuppliedalso with a wave which is harmonically related to and generatedfrom a respective one of the stable reference frequency waves. Forproviding an additional reference frequency wave, another oscillator isutilized, and by means of a pulse-locking arrangement including a pulsegenerator excited by the output of this same oscillator, such oscillatoris locked at a frequency which is a subharmonic of one of the referencefrequency waves. By means of a pulse-locking arrangement including thissame pulse generator, still another oscillator is locked at a frequencywhich is a harmonic of the additional reference frequency wavementioned, and a wave representative `of the output of this lastoscillator is fed into the last of the plurality of cascaded mixers. Theoutput of this last mixer is utilized (e. g., in a phase discriminator)as a wave 'representative of the output of the multichannel masteroscillator.

A detailed description of the `ininantion follows, taken in conjunctionwith the accompanying drawings, wherein:

Fig. 1 is a block diagram of a system utilizing this invention; and

Fig. 2 is a detailed circuit digram of a portion of the system ofFig. 1. 4

Referring now to Fig. 1, the multichannel master (captive) oscillator 1is the oscillator that is automatically controlled in frequency by thefrequency control system illustrated in this figure, and the output ofthis oscillator is utilized in the transmitter-receiver (not shown) withwhich the system illustrated is associated. A The transmitter-receivermay for example be arranged as disclosed in my aforesaid applicationSerial No. 257,148, now Patent No. 2,754,421 `dated July 10, 1956.However, in Fig. l there is provided a choice of (only) r22,000 possiblefrequency channels, spaced 500 cycles apart `in the range of '1.9 to12.9 me., for theoscillator 1. Thelmaster oscillator 1 is arranged to bepermeability tuned and has an output frequency of 1.9 to 12.9 mc. (inseveral bands), as indicated. Exact frequency'control of oscillator 1 isobtained 1-by means of a reactance tube 2 coupled infrequencyacontrolling relation to `oscillator 1. i i

A single very accurate and stable source of reference frequency Waves isprovided. The heart of the unit which provides these referencefrequencies is a reference crystalcontrolled `oscillator 3 whichoperates at a` frequency by utilizing, for feeding the last mixer of thecascade of one mc. and is extremely stable. Crystal 3 provides, by meansof a series of cascaded `fretpiency dividers, all

of the stable reference frequency Waves required. The` first frequencydivider 4 divides the frequency of oscillator 5 by two, to produce astable reference frequency of 500 kc, which drives a SGO-kc. harmonicgenerator 5. Generator 5 generates harmonics `of the SOO-kc. inputfrequency fed. thereto. `A thousands selection switch 6 has twenty-twopositions and is mechanically coupled to a frequency selecting means ingenerator 5 so that any selected one of the 6th through 27th harmonicsof the 50G-kc. input to` lgeneator 5 may be passed from said` generatorto No. 1 mixer 7, `dependingupon the posi-` tion of switch 6. vAnyselected one of the SOO-kc. harmonic frequencies between 3 and 13.5 mc.may be passed on to mixer 7. Thus, a reference frequency waveharmonically related 'to and generated from the SOO-kc. ref- Y erencefrequency wave (output of divider 4) is` fed 'from harmonic generator'Sinto mixer 7. Mixer 7 is the first in a plurality of cascaded mixers. I

Output from the master oscillator 1 is also supplied to the first mixerof 'the `series of cascaded mixers and this oscillator frequency,beating with the output frequency of generator 5 in such mixer, producesa difference frequency mixer output `which may vary from 600 to 1100kc., depending upon the settings of the frequency selection switches 6and '16.`

The 500-kc. output of divider 4 in turn excites a further series offrequency dividers, beginning with a 100- kc. divider 8 the output ofwhich drives a 50-kc. stage kc. stage 9 includes amplifier and pulseshaper circuits whereby 'S0-kc. pulses and a SO-kc. sawtooth wave may bederived from this stage for utilization in circuits to be laterdescribed. In addition, a frequency multiplier 11 providing amultiplication factor of three is coupled to receive a portion of theoutput of divider 8, thereby to produce a reference frequency wave of300 kc. for utilization in a circuit to be later described.

The 600-1l00 kc. difference frequency output of mixer 7 is passedthrough a bandpass iilter 12 t-o provide one of the inputs to No. 2mixer 13 (the second in the plurality of cascaded mixers), the otherinput to this mixer being provided from a 50-kc. harmonic generator 14.

The generator 14 is'supplied with 50-kc. pulse input derived fromdivider stage 9 over lead 15, and harmonics of this input frequencylying in the range of 450 to 900 kc. (to Wit, the range covered by the9th through 18th harmonics of the 50-kc. input frequency) are selectedVby the hundreds selection switch 16, which hasten positions. Theparticular harmonic of 50 kc. selected at the output of generator 14depends of courseupon the position of switch 16, and this selectedharmonic is passed on to mixer 13 to mix with signal from filter 12(mixer 7). Again, a reference frequency wave har-` monically related toand generated from the 50-kc. reference frequency wave (output ofdivider 9) is fed from harmonic generator 14 into mixer 13. Theselective circuit in lter 12 is tuned approximately by the hundredsswitch 16.

Output from mixer 13 is transferred, through the selective circuitbandpass filter 17, tunable in ten steps between 150 and 200 kc. as thetens switch 18 (which has ten positions) determines, to No. 3 mixer 19(the third inthe plurality of cascaded mixers). A kc. harmonic generator20 is supplied with4 5kc. input derived from divider stage 10, andharmonics of this input frequency lying `in the range of 35 to S0 kc..(to wit, the range covered by the 7th throughlth harmonics of the 5-kc. input frequency) are selected bythe tens switch 18. n The particularharmonic vof 5 kc. which is selected byswitch 18 from generator 20 ispassed on to mixer 19 as input to mix with signal from lter 17 (mixer13). Again, a reference frequency wave harmonically related to andgenerated from the 5kc. reference frequency wave (output of divider isfed from harmonic generator 20 into mixer 19.

Output from mixer 19 is transferred through the bandpass filter 21,which passes a frequency band from 230 to 235 kc., to No. 4 mixer 22(the fourth and last in the plurality of cascaded mixers).

The other input for mixer 22is obtained in part fr0 an oscillator 23,which is pulse-controlled in a manner to be described hereinafter tooperate at any selected one of a plurality of discrete frequenciesspaced apart 500 cycles, in the range of 30 to 35 kc. Oscillator 23 istunable (switchable) by means of a units selection switch 24, which hasten positions, to tune it to any one of the various discrete frequencies(spaced 500.' cycles apart) in the range of 30 to 35 kc. A portion ofthe output of oscillator 23 is fed as one input to'a phase discriminator25 having two inputs and a single output. The other input todiscriminator 25- is obtained from a pulse generator 26,-which suppliespulses occurring at a` stable'rate of 500 pulses per second (P. P. S.)to the discriminator. These pulses are locked to the reference crystal 3in a manner to be described hereinafter. The 500-P; P. S. output ofpulse generator 26 is fed as one inputv to phase discriminator 2,5, theother input to this discriminator being furnishedby the pulse-controlledoscillator 23. The output of discriminator 25 is applied @eraser '4 to areactance tube 27 which is coupled in frequencycontrolling relation tothe pulse-controlled oscillator 23, whereby the output of discriminator25 locks in the frequency of oscillator 23 by means of this reactancetube. In a vmanner to be described further hereinafter, using the phasediscriminator 25 and reactance tube 27, the oscillator 23 is locked orsynchronized to a selected harmonic of the 500-P. P. S. stable output ofgenerator 26, so that this oscillator (which may thus be termed apulse-controlled oscillator) provides an output ofv any selected one ofa plurality of predetermined frequencies spaced 500 C. P. S. apart. Thisis true since oscillator 23 may be synchronized to a series ofsuccessive harmonics of the pulse recurrence rate of pulse generator 26(which is 500 P. P. S.). Oscillator 23 is capable of being locked to anyone of the 60th through 69th harmonics of the 500-P. P. S. output ofpulse generator 26. Further details of the operation'of oscillator 23will` not be'given until later, so that the description of the over-allcontrol system can now be completed.

Another portion of the .output of oscillator 23 (in addition to thatportion of the oscillator output fed to discriminator 25) is fed as oneinput to No. 5` mixer 28. In mixer 28, the output of oscillator 23 ismixed with a SOO-kc. stable frequency wave fromY multiplier 11 toproduce output from this last mixer of any one of ten frequencies,spaced every 500 cycles in the range from 265 to 270 kc. A bandpassfilter 29 couples the output of mixer 28 to the last or No. 4 mixer 22.

The output of No. 4 mixer 22 is nominally 500kc. ln other words, as themaster (captive) oscillatorl is scanned through a'band of frequenciesthere will beV one segment of the oscillator tuning range', correspond.ing to the settings of the'switches 6, 16,- 18, and 24 (which determinethe selected frequencies fed to the several mixers) Where asignal near500kc..wil1 be devel; oped in the output of mixer 22; this signalVo'utputin the vicinity of 500 kc. corresponds closely to the desiredcorrect tuningof the master oscillator 1. The 500kc.output of'mixer'22is passed through a selective lter 30v (tuned to 500 kc.) to the inputof a regenerative-type frequency divider 31 having al division ratio often. Frequency divider29 ydivides the 500-kc. output of filter 30 (mixer22) down to 50 kc., and this 50-kc.. wave is coupled as one input tophase discriminator 32. A 50-kc. sawtoothf shapedoutputderived fromdivider stage 9 over lead 33 is supplied asthe other input to phasediscriminator 32.v In the phase detector or discriminator 32, a D. C.control' output results from the phase comparisonof the 50-ke. signalfrom divider 31 and the 50-kc. sawtoothsignal derived from `thereference 50-kc. source 9. Thel control output of the phasediscriminator 32 is direct coupled (preferablythrough a cathode followerstage, not shown) to the grid of the reactance tube 2 for the masteroscillator 1, in order-tocorrect for slow frequency drifts of the masterloscillator 1. Y

The system described constitutes an automatic vfre-` quency controlsystem for the -master oscillator 1, by means of which the masteroscillator is stabilized in frequency by a phase discriminator 28 whichcompares the" heterodyned output of oscillator 1 (heterodyned throughmixers '7,' 13, 19 and 22) with the divided output of the referencecrystal oscillator 3 (divided through dividers 4, 8, and 9). Thearrangement described conl stitutes a multi-channel frequencygeneratonproviding 22,000 channels for the master oscillator 1, onevchannel every 500 cycles in the frequency range extending from 1.9 to12.9 mc. Each frequency channel is selected by the setting of the fourVswitches 6, 16, 18, and 24.

According to this invention, certain frequency dividing stages (withtheir concomitant tubes and tuned .transformers) which would ordinarilybe required, have 'been eliminated. More particularly, thepulse-controlled oscila` 75,; later '23V -is required to generatetenfrequency channelsl between 30 and`35 kc., Ithe channels being`spaced 500 kc. apart. Instead of using frequency divider stages fordividing the reference frequency, available at kc. at the output ofdivider 10, to 500 cycl'esand then exciting the pulse generator 26 bythis SOO-cycle wave, an additional winding on the pulse transformer(which constitutes part of pulse generator 26) is provided, thisadditional winding feeding an additional phase discriminator 34. Thatis, one ofthe two inputs to phase discrimina-tor 34 is provided by the50G-P. P. S. output of pulse generator 26. The other input todiscriminator `34 is provided by the 5kc. (reference frequency wave)output of frequency divider 10.

The outputof phase discriminator 34 is fed to a frequency control tube35 which is coupled in frequency-controlling relation to an auxiliarySOO-C. P. S. oscillator 36, whereby the output of discriminator 34 locksin the frequency of oscillator 36 by means of this control tube. In amanner to be described further hereinafter, using the phasediscriminator 34 and control tube 35, the oscillator 36 is locked orsynchronized to a submultiple or subharmonic of the S-kc. stable outputof divider i0, and specifically to a frequency (500 C. P. S.) which is1/10 of the `5-kc. frequency output of divider l0. Oscillator `36 (whichmay be termed a locked oscilla-tor, since it is locked to a subharmonicof the 5-lic. frequency output of divider has `a stable frequencysubstantially sinusoidal output ofSOt) C. P. S., this outputbeingcoupled to the input of pulse generator 26 to provide theexcitation therefor. stantially sinusoidal input` wave to'tshort, sharppulses having the same periodicity or recurrence` rate as the sine waveexcitation, that is, having a recurrence rate of 500 P. P. S., one pulsebeing produced for each cycle of thev SOO-C. P.l S.` sine waveexcitation. The control loopincludingelements 36,26, 34, and is alockedoscillator type of frequency divider,` which` produces stable S-P.P. S.. pulses (at the output of pulse genera tor 26) from the 5-kc.stable reference frequency wave output of frequency divider 1t).

It may Ahe seen that the pulse generator 26 `supplies SOO-P. P. S.pulses to phase discriminator 34 (to enable the locking-in of oscillator36) and also to phase dis- `criminator 25 (to lock in or synchronizeoscillator 23).

Thus, the same pulse generator 26 is used for two func* tions-to dividethe 5kc. reference frequency and to control or lock in the 30435 kc.oscillator 23.

-2 discloses detailed circuitry applicable to items 34, 35, 36, 26, 25,27, and 23 of Fig. 1. In Fig. 2, the oscillator 36 includes a triodevacuum tube connected in a more or less conventional manner to operateas an RC-type oscillator whose output `frequency is intended to be closeto 500 C. P. S. and is substantially sinusoidal. The SOO-C. P. S. sinewave output of oscillator 36 is applied through a pair of capacitors 37and 38 to the first or `control grid of a gas tetrode 39 which tertodeis connected bymeans of a series `resistor 4t) and a capacitor 41 (inshunt across the anode-cathode path of tube 39) to provide a relaxationoscillator circuit. The time constant of the RC circuit 40,141(thercharging circuit for capacitor 41) is a little faster than theperiodicity of the SOO-C. P. S. wave suppliedto tube 39, so thatcapacitor 41 becomes fully charged between positive` excursions ofthevoltage applied tolthis grid. When the grid of tube tetrode 39 lires todischarge capacitor 4l rapidly through tube 39, thus completing thesawtooth voltage wave which is initiated by the charging of thecapacitor. The rapid discharge of capacitor 41 produces a short, sharppulse of current through resistor 42 (which is' connected from thecathode of tube 39 to ground), whichpulse is applied tothe primarywinding 43 of a pulse transformer 44. The components 39-44 comprise thepulse generator26. Since the fcapacitor 41 discharges each time Pulsegenerator26 converts its sub the control grid of p the grid of tube 39is driven positive (at a SOO-cycle rate), pulses having a recurrencerate of 500 P. P. S. are produced in push-pull (that is, so as to haveopposite polarities) at the two ends of each of the two secondarywindings 45 and 46 of pulse transformer 44. The `arrangement is suchthat positive pulses appear at the upper end of winding 45 and negativepulses at the lower end of this same winding; also, positive pulsesappear at the upper `end of winding 46 and negative pulses at the lowerend of this same winding.

In order to provide a control loop for oscillator 36, so as to enablelocking-in of such oscillator, a 5-kc. reference frequency wave from theoutput of frequency divider 10 is fed through a capacitor 47' to thephase discrminator 34, to which the ends of secondary winding 45 arealso connected. The phase discriminator 34 comprises four rectiiiers(for example, type 1N34A rectifiers) 48, 49, 50, and 51 connected in abridge arrangement. The two inputs to the phase discriminator 34 are theSGU-P. P. S. pulses (from pulse transformer 44, excited from oscillator36) and the 5-kc. reference frequency wave (from divider 1(3) The 5-kc.reference frequency wave is derived ultimately from a. highly stablereference crystal 3 and so does not vary. The anodecathode paths ofrectiers 4S and Sti are connected in series `across the secondarywinding 45 (the connection to the upper end of winding 45 beingV madethrough a resistor 52 and a capacitor S3 connected in parallel), and theanode-cathode paths of the other two rectiliers 49 andMSl are connectedin series, with this last series combination across the rst diode seriescombination. Capacitor 47 feeds the 5-kc. reference frequency wave(output of divider Alil) to the common junction 54 of the rectifier 48cathode and the rectiter 5d anode. lf desired, a suitable bias voltagemay be applied to junction 54. p

The output of phase discriminator 34 is taken from the common junction55 of the rectifier 49 cathode and the rectifier 5l anode, and a storagecapacitor 56 is connected from this point 55 to ground. Point 55 is alsoconnected through a resistor 57 to the grid of a frequency control tube(triode vacuum tube) 35. The anode of tube 35 is connected to the anodeof oscillator tube 36 through capacitor 37, and is also connectedthrough an RC circuit 58 to the grid of tube 36. The anode-cathode pathof tube 35 constitutes an equivalent resistance the extent of whichdepends, among other things, upon the mutual conductanceof tube 35 andhence may be influenced by the voltage `applied to` its grid.` Sincethis equivalent resistance is in the RC network of oscillator' 36, thefrequency `of the output of this oscillator is iniiuenced by thefrequency control tube 35, as well as by the components of circuit 58.

If the two pulses of opposite polarity producedat the ends of secondarywinding 4S are applied to the diodes, all four diodes will conductsimultaneously (since posi tive pulsesare produced at the upper end ofwinding 45 and negativepulses are produced at the lower end of thiswinding), and their elfects will cancel out at points 54 and .55. t

Under static conditions (with the substantially sinusoidal voltageoutput of divider lll applied to the input of the diode bridge arrangevment), the capacitor 56 (terminal 55 thereof) will charge 39 is drivenpositively, gas t to the D. C. voltage at point 454.

The condition existing when` all four diodes are conducting is ashort-circuit from point 54 to point 55.' In other words, thediodes actas switches which connect point `54 to point 55 at the peaks of thepulses appearing in winding 45, since the four diodes are caused toconduct simultaneously when thepulses appear in secondary winding 45.Since the substantially sinusoidal voltage output ofthe divider 10isapplied to point54, the .instant the pulses are applied tothe diodessome portion of this of the phase discriminator 34v salaries? sinusoidalvoltage wave will besampled and applied 'to terminal 55 4ofthe capacitor56, charging the capacitor to this value. If Vthe frequency of thesinusoidal alternating voltage output'of divider 10 is a whole multipleof the rate of recurrence of the pulses, the same point of the sine wavewill be sampled each time a pulse is applied for switching, and thecapacitor 56 will hold a constant charge. The capacitance of capacitor56 is sufficiently large that a practically ripple-free unidirectionalcontrol voltage occurs across such capacitor,` and the amplitude of thisdepends upon the relative phasing of the pulses and the sinusoidaloutput of divider 10 (that is, upon the amplitude of the sinusoidal waveat the time of the pulses, when sampling occurs). If the relativephasing of the 50011. P. S. pulses and the -kc. sinusoidal output ofdivider is slightly changed, a

different portion ofthe sinusoidal wave will be sampled and thecapacitor 56 will charge or discharge through the short-circuit pathfrom point 54 to point 55 (which path is established in the mannerpreviously described, when all four diodes are caused to conductsimultaneously due to the effect of the pulses), to the new value. Theaverage value of the unidirectional control voltage across capacitor 56is obtained when the SOO-P. P. S. pulses in winding 45 occur at exactlythat moment when the 5kc. sinusoidal alternating voltage output ofdivider 10 passes through zero. A very small deviation from this particular phasing causes a larger or smaller control voltage to bedeveloped across capacitor 56. 4

The unidirectional control voltage across capacitor 56 is applied to thegrid of the frequency control tube 35, to influence the frequency ofoscillator 36. lIf the frequency or phase of oscillator 36 changes anamount which is not too large, the phasing ofthe pulses (developed fromthe outputof oscillator 36) will change with respect to that of thesinusoidal voltage output of divider 10, causing' the control voltageacross capacitor 56 to bc altered, and the variation in the oscillatorfrequency is compensated.' Thus, the frequency of oscillator 36 re mainsequal toa submultiple or subharmonic of the frequency of the referencefrequency wave applied to phase discriminator 34, and derived ultimatelyfrom the reference crystal 3 by way of divider 1l). Oscillator 36 isthus controlled or locked in with respect to the 544C. referencefrequency wave derived from divider 10, so as to lock in at asubharmonic orsubmultiple of such 5-kc. reference frequency wave.Specifically, oscillator 36 is locked in to operate at a frequency of500 C. P. S., 1,40 the frequency of the reference frequency wave.

If this synchronization of oscillator 36 by the 5-kc. referencefrequency Wave has not occurred at the moment of switching on, therewill be a periodic variation of the voltage produced across capacitor56; in other words, an alternating control voltage is obtained by whichthe oscillator voltage is frequency modulated. If, during thismodulation, the oscillator frequency passes a value which is equal to asubmultiple of the 5-kc. reference frequency,` the oscillator frequencywill remain at this value. For example, this last value may be 500 C. P.S.

To recapitulate, in my aforementioned application Serial No. Y584,103,various pulse-locked or pulse-,con-

trolled oscillators are synchronized or controlled by im-'.

pulse generators. It is possible also, however, to synchronize a pulsegenerator (or, actually, the oscillator feeding such a pulse generator)with a reference frequency'wave derived from a crystal; this is what lisdone, according to this invention, for pulse generator 26 and oscillator36. This is done by feeding the control voltage supplied by the phasediscriminator 34 to control tube 35 Which influences the frequency ofthe oscillator 36 (and pulse generator 26). The frequency of the, lockedoscillator 36 being lower than that of the 5-kc. reference frequency(out ofdivideri10),:frequency'division takes place in whichtheyfrequency ratio is thek same. as the multiplication factor in the saidcopending application.

The previous description has explained how theoutput of the pulsegenerator 26 (50G-P. P. S. pulses) is used to enable the oscillator 36to be locked in at a frequency which is a submultiple or subharmonicofthe 5-kc. reference frequency wave output of divider 10. It `will now beexplained how the output of this same pulse generator 26 is used to lockin or synchronize oscillator 23 at a frequency which is a multiple orharmonic of the 500- P. P. S. output of pulse generator 26.

It has previously been described how positive 500-r P. P. S. pulsesappear at the upper end of winding 46 and negative SOO-P. P. S. pulsesappear at the lower end of this same winding. l v

A triode vacuum tube 23 with the LC oscillatory circuit 59 forms anoscillator. Tube 23 vprovides the pulsecontrolled oscillator which issynchronized 'or locked to harmonics of the SOO-P. P. S. pulses suppliedbyl pulse generator 26 and appearing in thesecondary winding 46. Thefrequency of the output of oscillator 23 is influenced by thereactance'tube v27, as well as-by the components of circuit 59. r

The units switch 24 controls the frequency of oscil-v lator 23 to set itclose to any selected one of a plurality of frequencies which areharmonics of 500 C.- P. S. and which lie between 30 and 35 kc. Thisfrequency selection is accomplished in any suitable way, as by switchingof a selected oscillatory circuit 59 into the oscillator.

An alternating voltage of the oscillator 23 frequency which leads inphase about with respect to thealternating anode voltage, is supplied tothe grid of tube 27 via the phase-shifting network'A C1, R1.V Thiscauses the anode current of tube 27 to be about 90 leading in phase withrespect-.to the anode voltage, and the impedance of tube 27, betweenanode and cathode, will have a reactive character. The extent of theequivalent reactance depends, among other things, upon the mutualconductancel of tube 27 and hence may be influenced by the voltageapplied to its grid.

A portion of the output of the pulse-control1ed oscillator 23 is takenoff from the oscillatory circuit 59 and fed through a capacitor 60 tomixer 28, for mixing therein with theBOO-kc. reference frequency waveand for later application to the last mixer 22 of the series of cascadedmixers. i

In order to provide a control lo'op for the oscillator 23, to make thisoscillator a pulse-controlled one, a sample of the oscillator output istaken off from the oscillatory circuit 59 and is fed through a capacitor61 tothe phase discriminator 25, to which the ends of vthe secondarywinding 46 are also connected. The phase discriminator 25 comprises fourrectiers 48', 49', 50', and 51' arranged just as in discriminator 34previously described and operating quite similarly. Elements of thephase discriminator 25 which are similar to those of phase discriminator34 are denoted by the same reference numerals, but vcarrying primedesignations. The two inputs to the phase discriminator 25 are the50G-P. P. S. pulses (from pulse transformer secondary winding 46) andthe output of the oscillator 23,. Y n

Point 55 (the ungrounded plate of the capacitor 56') is connected to thegrid ofa triode vacuum tube 62'connected as a cathode follower, and thevoltage across the cathode resistor 63 of this cathode follower circuitis applied tothe grid of reactance tube 27 through resistors 64 and R1.

Just as previously described in connection with phase discriminator 34,diodes 48', 49', 50', and 51' act as swtiches which connect point 54' topoint 55 at the peaks of the pulses appearing in winding 46. Sincethesubstantially sinusoidal voltage -output of the oscillator 23 isapplied to point 54', the instant the pulses are applied to the diodessome portion of this sinusoidal voltage wave will be sampled and appliedto terminal 55'y of the capacitor l56', charging the capacitor to'thisvalue. Iftherfre-` quency of the sinusoidal alternating voltage outputof oscillator 23- is a whole multiple of the rate of recurrence of thepulses,` the same `point `of the sine wave will be sampled each time apulse is applied for switching, and the capacitor 56 will hold aconstant charge. The capacitance of capacitor 56 is sufficiently largethat a practically ripple-free unidirectional control voltage occursacross such capacitor, and the amplitude of this depends upon therelative phasing of the pulses and the sinusoidal output of oscillator23 (that is, upon the amplitude of the sinusoidal Wave at the time ofthe pulses, when sampling occurs); If the relative phasing of the pulsesand the sinusoidal output of oscillator 23 is slightly changed, adifferent portion of the sinusoidal wave will be sampled and thecapacitor 56' will charge or discharge through the short-circuit pathfrom point 54 to point 55', tothe new value. The average value of theunidirectional control voltage across capacitor 56, is obtained when thepulses in winding 46 occur at exactly that moment when the sinusoidalalternating voltage output of oscillator 23 passes through zero. A verysmall deviation from this particular phasing causes a larger or smallercontrol voltage to be developed across capacitor 56.

The unidirectional control voltage across capacitor 56 is applied to thegrid of the D. C.coupled cathodefollower triode 62 andthe D. C. outputvoltage appearing across `cathode resistor 63 is applied to the grid ofthe reactance tube 27, to influence the frequency of oscillator 23. Ifthe frequency or phase of oscillator 23 changes an amount which is nottoo large, the phasing of the sinusoidal voltage output `of oscillator23 will change with respect to that of the pulses, causing the controlvoltage across capacitor 56' to be altered, and the variation in theoscillator' frequency is compensated. rl`hus, the frequency ofoscillator 23 remains equal to a whole multiple of the recurrencefrequency of the 50G-P. P. S. pulses applied to phase discriminator 25,and derived from the locked oscillator `36. Oscillator 23 is thuscontrolled by the pulses derived from pulse generator 26, so that it maybe locked or synchronized at various harmonics of the 500- P. P. S.recurrence frequency of `the pulses providedby generator 26. Thesepulsesare in turn developed from the stable,frequencyoutput of lockedoscillator 36, as previously` described.,- i

If this synchronization of oscillator 23 by the pulses has` notoccurredat the moment .of` `switching on, there will be a periodic variation ofthe voltage produced across capacitor 56'; in` `other Words, analternating control voltage is obtained by which the oscillator voltageis frequency modulated. If, during this modulation, the oscillatorfrequency passes a value which is equal to a whole multiple of the pulsefrequency, the` oscillator frequency will remain at this value.

Synchronization of the oscillator 23 may occur when the ratio betweenits frequency and the pulse recurrence frequency (500 P. P. S.) is alarge integer. Synchronizationup to the 70th harmonic of the pulserecurrence frequency (as given hy way of example for oscillator 23,which is synchronized at as high as 35 kc. from a 500- P. P. S. pulsesource) is easilyachieved.

Itfmay be seen that the pulse generator 26 supplies 50G-P. P. S. pulsesto phase discriminator 34 (toenable locking-in of oscillator 36 andconsequent frequency divisionzof the S-kc.` reference frequency wavefrom divider and also `to phase discriminator 25 (to lock` in orsynchronizeoscillator 23 at a harmonic of the SOO-P.A P. S. pulse rate).

What is claimed is: i.

`l. In a `frequency control system for a multichannel oscillator, `asingle stable frequency source providing a plurality of referencefrequency waves, `n mixers arranged in Jcascade,`where nis more than l,means feeding a sample of the wave `output of said oscillator into thefirst of said mixers, (nal) means for feeding a reference frequency wave"harmonically related to a respective one of said first-mentioned wavesinto each respective mixer save the last, means feeding the output ofeach respective mixer save the last to the input of the next successivemixer, a second oscillator, a pulse generator coupled to the output ofsaid second oscillator for developing from such output a series ofpulses having a recurrence frequency equal to the frequency of thesecond oscillator output, a first phase discriminator having two inputsand an output, means coupling the output of said pulse generator to oneof said two inputs, `means for applying one of said reference frequencywaves to the other of said two inputs, means coupling the output of saidfirst discriminator to a frequencycontrolling device for said secondoscillator, a third oscillator, a second phase discriminatorhaving twoinputs and an output, means couplingk the output of said pulse generatorto one of the `inputs of said second discriminator, means coupling theoutput of said second discriminator to a frequency-controlling devicefor said third oscillator, means coupled to said third oscillator forfeeding a Wave representative of the output of said third oscillatorinto the last mixer, andmeans for utilizing the output of said lastmixer as a wave representative of the output of said multichannel`oscillator. i i

2. In a frequency control system for a multichannel oscillator, a singlestable frequency source providing a plurality ofreference frequencywaves, n mixers arranged in cascade, where nis more than l, meansfeeding a sample of the wave output of said oscillator into the first ofsaid mixers, (n-l) means for feeding a reference frequency waveharmonically related to arespective one of said `first-mentioned wavesinto each respective mixer save the last, means feeding the output ofeach respective mixer save the last to the input of `the next successivemixer, a second oscillator, a pulse generator coupled to the output ofsaid second oscillator for developing` from such output a series ofpulses having a recurrence frequency equal to the frequency of thesecondoscillator output, a rst phase discriminator having two `inputs and anoutput, means coupling the output of said pulse generator to one of saidtwo inputs, means for applying one of said reference frequency wavestothe other of said two inputs, means coupling the output of said iirstdiscriminator to a frequencycontrolling device for said secondoscillator, whreby said second oscillator is locked to a frequencyharmonically related to that of said last-mentioned one referencefrequency wave, a third oscillator, a second phase discriminator havingtwo inputs and an output, means coupling the output of said pulsegenerator to one of the inputs of said second discriminator, meanscoupling the output of said third oscillator to the other of the inputsof said second discriminator, means coupling the output of said seconddiscriminator to a frequency-controlling device for said thirdoscillator, whereby said third oscillator is locked to a frequencyharmonically related to `that of said second oscillator, means coupledto said third oscillator for feeding a wave.. representative oftheoutput `of said third oscillator into the last mixer, and means forutilizing the output of said last mixer as, a wave representativeof theoutput-of `said multichannel oscillator. t

3. Ina,` frequency" control system for` a multichannel oscillatonwasingle stable frequency source providing a plurality of referencefrequency waves, n mixers arranged in cascade, where n is more than l,means feeding a sample ofthe wave output of said oscillator into thefirst of said mixers, (rz- 1) means for `feeding a reference frequencywave harmonically related to a respective one of said first-mentionedwaves into each respective mixer save the last, means feeding the outputof each respective mixer `save the last to the input of the nextsuccessive mixer, a second oscillator, a pulse generator coupled to theoutput of said second oscillator for developing from suchoutput `aseries of pulses having a recurrence frequency equal to `the frequencyof` the second oscillator output,` a first phase discriminator havingtwo inputs and an output,'means coupling the output of said pulsegeneratorgto one of said two inputs, means for applying one of saidreference frequency waves to the vother of said two inputs, meanscoupling the output of said'rst discriminator to a frequency-controllingdevice for said second oscillator, whereby said second oscillator islocked to a frequency which is a submultiple of that ofsaid-lastmentioned one reference frequency wave, a third oscillator, asecond phase discriminator having two inputs and an output, meanscoupling the output of said pulse generator to one of the inputs of saidsecond discriminator, means coupling the output of said third oscillatorto the other of the inputs of said second discriminator, means couplingthe output of said discriminator to a frequency-controlling device forsaid third oscillator, whereby said third oscillator is locked to afrequency which is a multipleof that of said second oscillator, meanscoupled toY said third oscillator for feeding a wave representative ofthe output of said third oscillator into the last mixer, and means forutilizing the output of said last mixer as a wave representative of theoutput of said multichannelv oscillator. Y

4. Ina frequency control system for a captive oscillator, a source of awave of reference frequency, a -second oscillator, a pulse generatorcoupled to the output of said second oscillator for developing from suchoutput a series of pulses having a recurrence frequency equal to thefrequency of the second oscillator output, a rst phase discriminatorhaving two inputs and an output, means coupling the output of said pulsegenerator to one of saidV two inputs, means'for applying said wave tothe other of said two inputs, means coupling the output of said firstdiscriminator to a frequency-controlling device for said secondoscillator, whereby said second oscillator is locked to a frequencywhichA is a submultiple of that of said wave, a third oscillator,` asecond phase discriminator having two inputs and an output, meanscoupling the output of said pulse generator to one of the inputs of saidsecond discriminator, means coupling the output of said thirdvoscillator to the other of the inputs of said second discriminator, andmeans coupling the AOutput of said second discriminator to afrequency-controlling device for said third oscillator, whereby saidthird oscillator is locked to a frequency which is a multiple of that ofsaid second oscillator.

5. In a frequency control system for a multichannel oscillator, a singlestable frequency source providing a plurality of reference frequencywaves, n mixers arranged in cascade, wherein n is more than one, meansfeeding a sample ofthe wave output of said oscillator into the rst ofsaid mixers, (n-l) means for feeding a reference frequency waveharmonically related to a respective one of said first-mentioned wavesinto each respective mixer save the last, means feeding the output ofeach respective mixer save the last to the input of the next successivemixer, a second oscillator for producing a first wave whose frequency isharmonically related to 'one of said reference frequency waves, anautomatic phase control Vloop including a phase discriminator andafrequency control device for'phase locking said second oscillator inharmonic relation with said vlast-mentioned one reference frequencywave; a third oscillator phase locked to the output of said secondoscillator for producing a` second wave whose frequency is harmonicallyrelated to that of said rst wave, means for heterodyning the output ofsaid third oscillator with a wave derived v4from one of said referencefrequency waves to produce anA altered frequency V'wave, means forfeeding said altered frequency wave into the last mixer, and means forutilizing the output of said last mixer as a wave representaltive of theoutput of said multichannel oscillator.

- 6. iA frequency control system fora multichannel oscillator, asinglest able frequency source providing a plurality of 'reference' frequencywaves, n mixers arranged inV cascade, wherein more than one, meansfeedingy a sample of the waveoutput of saidk oscillator into the firstof Vsaid mixers, (rt-1) means for feeding :a reference frequency waveharmonically related to a respective one of said first-mentioned wavesinto each respective mixer save' the last, means feeding the output of`each respective mixer save the last to the input ofthe next successivemixer, a second oscillator for producing a first wave whose frequency isa submultiple of one of said reference frequency waves, an automaticphase control loop includingY a phase discriminator iand aY frequencycontroldevice'for phase locking said second oscillator as a subharmonicof said last-mentioned one reference frequency wave; a third oscillatorphase locked to the output of said second oscillator for producing asecond wave whose frequency is a multiple-of that of said rstWave,lmeans for heterodyning the output of said thirdoscillatorwith awave vderived from one of said referenceV yfrequency waves to produce analtered frequency wave, means for feeding said altered frequency wavevinto the last mixer, and means -for utilizing the output of said lastmixer as a wave representative of the output of said multichanneloscillator. l A i 7.` In a frequency control system for a multichanneloscillator, a single stable frequency source providinga plurality ofreference frequency waves, n mixers arranged in cascade, wherein n ismore than one, means feeding a sample of the wave output of saidoscillator into the rst of said mixers, (r1-l) means for feeding areference frequency wave harmonically related to a respective one ofsaid first-mentioned waves into each respective mixer save the last,means feeding the output of each respective mixer save the last to theinput of the next successive mixer, a second oscillator, means receptiveof the output of said second oscillator for v developing therefrom aseries of pulses having a recurrence frequency equal to the frequency ofthe second oscillator output, an auto matic phase control loop includinga phase discriminator receptive of said pulses and of one of saidreference frequency waves, and including also a frequency controldevice, for phase locking said second oscillator in harmonic relation`withsaid last-mentioned one reference frequency wave; a thirdoscillator, means receptive of said pulses for locking said thirdoscillator to a frequency hormonically related to that of said secondoscillator, means for heterodyning the output of said third oscillatorwith a wave derived frompone of said'reference frequency waves toproduce an altered frequency wave, means for feeding said alteredfrequency wave into the last mixer, and means for utilizing the output0f said last mixer as a wave representative of ther output of saidmultichannel oscillator. y 8. In a frequency control system for amultichannel oscillator, a single stable frequency `source providing aplurality of reference frequency waves, n mixers arranged in cascade,wherein n is more than one, means feeding a sample of the wave output ofsaid' oscillator into the first of said mixers, (zz-l) means for feedinga reference frequency wave harmonically related to a respective one ofsaid first-mentioned waves into each respectivemixer save the last,means feeding the output of each respective mixer save the last to theinput of the next successive mixer, a second oscillator, means'receptiveof the loutput of said second oscillator for developing therefrom aseries of pulses having a recurrence frequency equal to the frequency ofthe second oscillator output, an automatic phase'control loop includinga phase discriminator 'receptive of said pulses and of one of saidreference frequency waves, and including also a frequency controldevice, for phase locking said second oscillator as a subharmonic ofsaid last-mentioned one reference frequency wave; a third oscillator,Imeans receptive of saidpulses for locking said 'third oscillator to afrequency which is a multiaereas? ple of that of said second oscillator,means for heterodyning the output of said third oscillator with a wavederived from one of said reference frequency waves to produce an alteredfrequency wave, means for feeding said altered frequency wave into thelast mixer, and means for utilizing the output of said last mixer as awave representative of the output of said multichannel oscillator.

l References Cited in the file of this patent UNITED STATES PATENTSGuanella Apr. 25, 1950 MacSorley Ian. 8, 1952 Marby Aug. 4, 1953Robinson Aug. 28, 1956 Israel Dec. 25. 1956

